Safety of Diagnostic Medical Sonography, Thermal and Mechanical Bioeffects

Introduction

Also known as ultrasonography, medical sonography is a diagnostic technique used in medical imaging, for purposes of visualizing muscles, tendons, as well as other internal organs, including their structure, size, and any pathological lesions that could be present. All this is done with the help of real time’tomographic images’. Furthermore, medical sonography has also been utilized in the visualization of a fetus, both during emergency and routine prenatal care.

With a wide application in the medical field, sonography is a portable and inexpensive technique, relative to such other techniques as computed tomography (CT) and magnetic resonance imaging (MRI). Up to this poet, there are no known risks that sonography, as a medical diagnostic technique poses to a patient. As such, it has often been described as ‘a safe test’ seeing that the technique does not utilize ionizing radiation, and which have been attributed to the causation of such hazards as the breakage of chromosomes, as well as cancer production.

Ultrasound heat effects

There are two probable physiological effects that have thus far been linked to ultrasound energy. First, ultra sound energy boosts inflammatory response, with a possibility of heating up the softer tissues (AIUM, 2008). According to an article published by the institute of ultrasound medicine off American (AIUM) on may 23, 2008, and which was titled ‘consensus report on potential bioeffects of diagnostic ultrasound’., this report asserted that indeed there are some probable dangers on a fetus when an ultrasound test gets administered. These possible dangers include “fetal thermals effects, postnatal thermal effects, fetal mechanical effects, postnatal mechanical effects, as well as bioeffects deliberations for contrasting agents of the ultrasound system” (AIUM, 2008).

Ultrasound energy is reported to assists in the production of a wave of mechanical pressure that passes though soft tissues. As a result, microscopic bubbles could come about in such soft tissues, leading to cell membrane getting distorted, thus persuading intracellular activity and ion fluxes (O’Brien & Zachary, 1997). Once ultrasound finds its way into the body, it influences friction at the molecular level, and this causes the tissues to get heated, albeit slightly.

This is an extremely minor effect, given that perfusions of normal tissues also dissipate heat. However, minute gas pockets could occur in tissues of body fluids at a higher intensity, with the effect that such affected tissues may expand and collapse, an occurrence that is referred to as cavitation.

Following an exposure to a hot bath of laboratory animals, it has been shown that a rise in birth defects amongst their offspring’s often occurs. Perhaps this is the reason why pregnant women are often advised to avoid hot water baths and saunas. It has not however been indicated just how much of heat should be considered as risky, with such a risk being dependent on the exposure time, as well as he developmental stage of a fetus. It has been shown that some ultrasound diagnostic equipments could lead to a rise in temperature of up to six degrees Celsius, on that spot at which the operator has focused the machine (Roy et al, 1990).

For this reason then, there is a need to ensure that an ultrasound probe is in a continuous motion, as a cushioning effect against localized elevation in temperature. A study that sought to reveal the effects of ultrasound on newborns found out that the techniques led to a 1.3 degrees Celsius rise in temperature, while at the same time causing a rapid blood circulation into the brain of an infant.

Upon exposure to a heat stressing factor, the cells of mammals are able to produce proteins resistant to heat. It is the position of scientists that these proteins so produced by mammals somewhat assist in protecting the cells against heat damage (O’Brien & Zachary, 1997). Nevertheless, the heating of the tissues by ultrasound technique is so rapid that cells do not have ample time to synthesize the heat-shock proteins.

Mechanical Effects

Besides heat, medical researchers and scientists have also embarked on studies to review the probable mechanical effects of the use of the ultrasound technique on the body. To this end, the effects have been categorized into two; the first kind has been termed acoustic cavitation. There is a probability for cavitation to occur following the passage of sound through a cavity-containing area, for example, air pockets or gas bubble (Roy et al, 1990).

Some tissues, especially the intestines and lungs of adults, have been shown to contain bubble of air. As such, these tend to be more vulnerable to the effects of cavitation. There are no apparent air bubbles on the intestines and lung of fetuses since fetuses relies on oxygen supplied from the blood stream of its mother. This notwithstanding, it is the position of researchers that the presence of minute bubbles of air have a likelihood of forming in other body parts, besides the intestines and lungs (Roy et al, 1990). This thus calls for further research into this area.

Sound waves in cavitation cause pockets of air or bubble to contract and expand steadily, meaning that these resonates, or pulsate. As a result, secondary sound waves are relayed by these bubbles in virtually all directions (Riesz & Kondo, 1992). Should the bubbles so much as result in a collapsing, this could lead to extremely elevated pressures and temperatures for a short time, say several tens of nanoseconds.

Due to the temperature and pressure build up in the tissues, free radicals could result, not to mention several other compounds that could be potentially toxic (Morgan et al, 1998). Although the occurrence of such compounds has been regarded as rather remote, they nevertheless have a potential, theoretically, to cause damage to the genetic make up of humans.

Besides, a fast bubble contraction during cavitation could lead to the emergence of liquid microjets, with a potential of damaging the cells. Among the laboratory animals, focusing diagnostic ultrasound on either the intestine or lung that possesses bubble of air could lead to the minute blood vessels rupturing, due to the effects of cavitation (Roy et al, 1990).

In addition, there are other mechanical effects that diagnostic ultrasound has been shown to create, and these may not need air bubbles for them to occur. Such effects entail pressure changes, torque (responsible for the rotation of things), force, and streaming (that is, liquid stirring). Sequentially, these changes could lad to clear sounds, redistribution and movement of cells in liquid, electrical alterations of cell membranes resulting in their increased permeability to bigger molecules, and cell damage (O’Brien & Zachary, 1997).

The passage of ultrasound through liquid medium results in acoustic streaming; which is more of a stirring action. Following a rise in the ultrasound acoustic pressure, there is a consequent increase in liquid flow (Apfe, 1982). In theory, this stirring action may as well take place the body parts of a patient that are filled with fluid. These include the amniotic sac, blood vessels, or even the bladder.

Animal experiments have revealed that following an encounter of liquid streaming with an object that is solid, there is a possibility of shearing action to take place, in effect leading to platelets damage, and eventually causing an abnormal clotting of blood (O’Brien & Zachary, 1997). Nevertheless, it is yet to be clarified the extent to which such a effect could occur among humans, following an exposure to the technique of diagnostic ultrasound.

Already, some studies have drawn an association between ultrasound and a rise in fetus movement during the time of scanning such a fetus. In trying to explain this phenomenon, one theory posits that the movement of the fetus is as a result of sound perceptions due to ultrasound beam pressure on encountering the bones at the head of a fetus (AIUM, 2008). Currently though, no evidence exists to suggests that such hearing sounds encountered during fetal scanning exercise pose any potential danger to a fetus.

Conclusion

Diagnostic medical sonography has been utilized in medical imaging for a long time now. Owing to its portability and flexibility, sonography has found wise application in the medical field. Up to now, there are no apparent risks that research into the technique seems to attribute to its recipients.

Nevertheless, scientists have not ruled out mechanical and heat effects of the technique on patients. One effects is referred to as acoustic cavitation, and involves an expansion and contraction of body tissues, with a consequent build up in pressures and temperature, and this lead to the development of free radicals which are theoretically capable of causing genetic damage.

In addition, a rapid heating up of the tissues, as happens when the sonography technique is being utilized, often inhibits the production of heat-shock proteins, and which prevents cell damage. As such, the sonogreaphy techniques could lead to damaged cells, but there is a need for more research to be undertaken into this area.

References

American Insitute of Ultrasound in Medicine (AIUM) bioeffects committee. Bioeffects and safety of diagnostic ultrasound in medicine, 2008.

Apfel, R. E. “Acoustic cavitation: a possible consequence of biomedical uses of ultrasound”. British journal of cancer, 45 (1982):140

Morgan, T.R., Laudone, V. P., & Heston, W. D. “Free radical production by high energy shock waves: comparison with ionizing irradiation. Journal of Urology, 139 (1988): 166.

O’Brien, W. D, & Zachary, J. F. “Lung damage assessment from exposure to pulsed- wave ultrasound in the rabbit, mouse, and pig. IEEE Trans Ultrasonics, ferroelectrics, and frequency control, 44 (1997): 473.

Riesz, P, & Kondo, T.” Free radical formation induced by ultrasound and its biological implications”, Journal of free radical and biological medicine, 13 (1992):247.

Roy, R. A., Madanshetty, S.I & Apfel, R. E. “An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound. Journal of acoustic science and medicine, 87 (1990): 3452.